refs #115647: optional disable Buffer::invariant check
refs #115647: optional disable Buffer::invariant check

* change Buffer::invariant() with NDEBUG_PERF so that application can
  optionally disable the expensive invariant check.

Change-Id: I721f55c06a8aff5a3a941852f160abcb017cde5d
Signed-off-by: Kevin Cai <c4f49ef5a44534d46de426dda18a118a2ca4debf@mozy.com>
Reviewed-on: https://gerrit.dechocorp.com/63230
Reviewed-by: Jenkins <49695b0f9c673fd73dab0ff3f84b5ced9302cc6a@mozy.com>
Reviewed-by: Kefu Chai <ef1142690fdf7c5d47bbd7e2a7a410478308cc21@mozy.com>
Reviewed-by: Andrew Skowronski <96924f7a4dbe24093bb2da18f50a76703cb4a362@mozy.com>

mozy/mordor,mozy/mordor,mozy/mordor